[Fleet] Fix cancel action in log level select

[criamico]

Summary

Part of #158861
#180607 added a new log level selector in Agent Policy settings. However there were some small bugs with it. This PR addresses all of them:

• Extra dot in copy text
• A broken link was displayed - Made this link optional
• When clicking "Cancel", all the other fields on the page reset back to their original values but the log level doesn't. The reason is the no default was set for the select

Before

After

Testing

• Enable hidden: false here
• Change the log level and then click "cancel" on the bottom of the page, it should reset back to the default value ("info" if the policy is new, the previous saved value if the policy already had this value set)
• No extra dot and link should be visible

[apmmachine]

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[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[juliaElastic]

LGTM

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 477a3db

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
fleet	1.3MB	1.3MB	+77.0B

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @criamico